Thick grain-oriented electrical steel sheet exhibiting excellent magnetic properties

ABSTRACT

A grain-oriented electrical steel sheet comprising 2.5-4.5% Si by weight and measuring 0.36-1.00 mm in thickness is imparted with a good core loss value for its thickness by controlling its C content, flux density, grain boundary configuration, and deviation degree of crystal orientation in the grains.

CROSS-REFERENCING

[0001] This application is a continuation-in-part under 35 U.S.C. §120of prior application Ser. No. 08/116,152 filed Sep. 2, 1993, the benefitof which is claimed. The disclosure of the specification, claims,abstract and drawings of prior application Ser. No. 08/116,152 filedSep. 2, 1993 is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to a thick grain-oriented electrical steelsheet exhibiting excellent magnetic properties and suitable for use asthe material for the core of a transformer or the like.

[0004] 2. Description of the Prior Art

[0005] Since grain-oriented electrical steel sheet is used mainly as acore material for transformers and other electrical equipment, it isrequired to exhibit excellent magnetic properties, most notablyexcellent magnetization property and core loss property. Magnetizationproperty is generally expressed as the flux density B₈ value at amagnetic field of 800 A/m, and core loss property is expressed as theW_(17/50) core loss value at a frequency of 50 Hz and a magnetization to1.7 Tesla.

[0006] The main factor governing core loss property is flux density.Generally speaking, the higher the flux density, the better is the coreloss property. Notwithstanding, increasing the flux density causes thesecondary recrystallization grain size to be enlarged simultaneouslyand, to the extent that it does, has a degrading effect on the core lossproperty. In contrast, magnetic domain control enables an improvement incore loss property irrespective of the secondary recrystallization graindiameter.

[0007] Grain-oriented electrical steel sheet is produced with use ofsecondary recrystallization phenomenon in the final annealing step so asto develop a Goss texture wherein the grains have their (110) axesaligned with the sheet surface and their <001> axes aligned with therolling direction. For obtaining good magnetic properties, the easilymagnetizable <001> axis has to have a high degree of alignment with therolling direction.

[0008] JP-B-40-15644 and JP-B-51-13469 teach typical methods forproducing a high flux density grain-oriented electrical steel sheet.JP-B-40-15644 describes a method using MnS and AN as the main inhibitorsand JP-B-51-13469 describes a method of using MnS, MnSe, Sb and the likeas the main inhibitors. Appropriate control of the size, morphology anddistribution of the precipitates functioning as inhibitors is thereforean indispensable requirement in the currently available technology.

[0009] On the other hand, owing to the desire of transformermanufacturers to increase the energy efficiency and lower the cost oftheir products, the laminated core sector has experienced increasingneed for thick grain oriented electrical steel sheet enabling areduction in the number of laminations. Moreover, the large rotatingmachine sector has also long showed an interest in using grain-orientedelectrical steel sheet. Here again the need is particularly high forthick grain-oriented electrical steel sheet that allows the number oflaminations to be reduced.

[0010] Since increasing sheet thickness generally leads to degradationof core loss property, a strong need has arisen for the development of athick grain-oriented electrical steel sheet with excellent magneticproperties.

SUMMARY OF THE INVENTION

[0011] The object of this invention is to provide a thick grain-orientedelectrical steel sheet exhibiting good magnetic properties.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a graph showing how the core loss property of a productsheet is affected by its carbon content and flux density.

[0013]FIG. 2 is a graph showing how the core loss property of a productsheet is affected by the shape factor of the grain boundary and thedeviation degree of crystal orientation in the grains.

[0014]FIG. 3 is a graph showing how the core loss property of a productsheet is affected by sheet thickness, in products according to theinvention and in comparison products.

[0015]FIG. 4 shows a typical grain pattern of a thick grain-orientedelectrical steel sheet according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0016] This invention provides a grain-oriented electrical steel sheetwith excellent magnetic properties, the electrical steel sheetcontaining 2.5-4.5% Si by weight, measuring 0.36-1.00 mm in thickness,having a C content of not greater than 0.0050% by weight, exhibiting amagnetic flux density B₈ of not less than 1.83 T, exhibiting anSF(average value) of less than 0.80, where SF is an index representingthe boundary configuration characteristics of the individual sheetgrains with the same area as the circle with diameter exceeding 5 mm hasand is defined as

[0017] SF=(grain area×4 π)/(grain boundary length )², the SF (averagevalue) being the average value of the individual SF values, its grainsof a diameter exceeding 5 mm having a crystal orientation deviation of0.2-4 degrees in relation to the crystal orientation at the grain centerof gravity, and, as a product sheet of a thickness t (mm), exhibiting acore loss W_(17/50) (w/kg) of not more than 3.3×t+0.35.

[0018] The grain-oriented electrical steel sheet of the presentinvention is produced by sequentially conducting the steps of castingmolten steel obtained by a conventional steelmaking method eithercontinuously or by the ingot making method, if the ingot making methodis used slabbing the ingot to obtain a slab, hot rolling the slab toobtain a hot-rolled sheet, annealing the hot-rolled sheet as required,subjecting the sheet to a one stage cold rolling or two or more stagesof cold rolling with intermediate annealing, decarburization annealingthe cold-rolled sheet, and subjecting the decarburized sheet to finalfinish annealing.

[0019] The inventors made a broad-based study of the conditions requiredfor realizing good magnetic properties in the process for producingthick grain-oriented electrical steel sheet. This enabled them toascertain the requirements that must be met by the product.

[0020] JP-A-3-72027 and U.S. Pat. Nos. 3,969,162, 4,054,471 and4,318,758 describe methods of manufacturing grain-oriented electricalsteel sheet.

[0021] In the prior art, manufacturers focused on reducing the thicknessof the sheets in order to improve the core-loss properties of thegrain-oriented electrical steel sheet. Specifically, it shows a thinningof such sheet from 0.35 mm to 0.23 mm.

[0022] In the manufacturing methods of the prior art, for metallurgicalreasons (high temperature slab heating), in the initial phase (the meltpreparation step), it was necessary to have a carbon content that washigher than required. In the final product, it was necessary that thecarbon content did not exceed 0.0050%, so decarburization was includedas an intermediate step. Because the ease of the decarburization wasinversely proportional to the thickness of the sheet and, moreover,dense oxidation layers were formed during the decarburization, it wasnecessary to submit thick sheet (over 0.35 mm) to two decarburizationpasses, between which pickling had to be used to remove oxidation layerspossessing decarburization-difficulty. This greatly increased themanufacturing costs, making the manufacturing process commerciallyunfeasible. For example, because the difference between 0.35 mm and 0.40mm is the square of the thickness, the difference in terms ofdecarburization properties was 31%, rather than the 14%.

[0023] However, in the manufacture of transformers, the steel cores ofthe transformers are formed by stacking layers of grain-orientedelectrical steel sheet to achieve the required core size. This meansthat it takes less work if the sheets used are thicker, enhancingproductivity. However, because of the reasons cited above, using thickersheets is too costly to be practical. For transformer manufacturers, thetrade-off point between manufacturing ease (including cost) and thedesire for thicker sheets was 0.35 mm, which was the thickness used incommercial products in the prior art. That is, mathematically, there islittle difference between 0.35 mm and 0.36 mm, but in terms of themethod of manufacturing the grain-oriented electrical steel sheets, 0.36mm was the critical point. In terms of use by customers, there is atremendous difference between product sheet up to 0.35 mm and productsheet that is 0.36 mm or thicker. In terms of the process ofmanufacturing transformers, compared to 0.35 mm sheets, using 0.40 mmsheet reduces the number of stacking steps by 14%, and use of 0.50 mmsheets reduces the number of steps by 43%. Since transformers is alabor-intensive industry, being able to reduce the number of stackingsteps by over 10% is extremely valuable.

[0024] Their findings will now be explained with reference toexperimental results.

[0025]FIG. 1 shows the effect of the product C content and flux densityon the product core loss property.

[0026] In this experiment, a silicon steel slab comprising, by weight,3.21-3.30% Si, 0.025-0.085% C, 0.025-0.030 % acid-soluble Al,0.0075-0.0086% N, 0.070-0.161% Mn, 0.005-0.029% S and the balance Fe andunavoidable impurities was heated at 1150-1380° C. for 1 hr, the slabwas hot rolled into a 2.8 mm-thick hot-rolled sheet, one portion of thehot-rolled sheet was annealed at 900-1100° C. and another portionthereof was not annealed, and the sheets were cold rolled at a reductionratio of about 83% to a thickness of 0.48 mm.

[0027] The so-obtained cold-rolled sheets were subjected todecarburization annealing (atmosphere: 25% N₂ and 75% H₂; dew point: 65°C.) in the temperature range of 810-860° C. for 250 sec. Then, a portionof each sheet was subjected to nitriding treatment, with which N wasincreased by 0.0102-0.0195%, using NH₃ gas during 750° C.×30 secadditional annealing and another portion of each sheet was not subjectedto nitriding treatment. The sheets were coated with an annealingseparation agent consisting mainly of MgO, the coated sheets were rolledinto (5-ton) coils measuring 200-1500 mm in inside diameter, the coilswere subjected to final finishing annealing by heating to 1200° C. at atemperature increase rate of 15° C./hr in an annealing atmospherecontaining 10-100% N₂ (remainder H₂), and by holding them at 1200° C.for 20 hr in an H₂ annealing atmosphere.

[0028] The coils were applied with a tensile coating and then cut to asize for a single sheet tester, flattened, maintained at 850° C. for 4hr for strain relieving annealing, whereafter the magnetic propertieswere measured. The final product thickness was 0.50 mm.

[0029] As is clear from FIG. 1, products exhibiting a good core lossproperty, namely a W_(17/50) of not greater than 2.00 w/kg, wereobtained only under the conditions of a carbon content of not more than0.0050% and a flux density B₈ of not less than 1.83 T. Even when theseconditions were met, however, there were cases where the W_(17/50) wasgreater than 2.00 w/kg. The reason for this was carefully investigated.

[0030] The results of the investigation will be explained in thefollowing.

[0031]FIG. 2 relates to those among the products of the test of FIG. 1which had a carbon content of not more than 0.0050% and a flux densityB₈ of not less than 1.83 T and shows how the core loss property of theseproducts was affected by the shape factor (SF) of the grain boundary ofgrains with the same area as the circle with diameter exceeding 5 mm hasand the deviation degree (Δθ) of crystal orientation in grains of adiameter exceeding 5 mm.

[0032] The grain boundary shape factor (SF) was defined as SF=(grainarea×4 π)/(grain boundary length)², and used to quantify grain boundaryconfiguration. The value of SF is 1 for a circular grain and decreaseswith increasing irregularity (bumpiness) of the grain boundaryconfiguration.

[0033] The deviation degree (Δθ) of crystal orientation in grains of adiameter exceeding 5 mm represents the difference in orientation in thegrain in relation to that at the grain center of gravity. When, as inthe present invention, secondary recrystallization is evolved in thecoiled state and the coil is thereafter flattened to provide theproduct, the crystal orientation deviation (Δθ) in the grains generallytends to increase with increasing distance from the center of gravity inthe rolling direction.

[0034] SF was measured by image analysis and Δθ was measured usingElectron Channeling Pattern (ECP).

[0035] Each dot in FIG. 2 corresponds to an SST-sized specimen producedunder the experimental conditions of FIG. 1. SF is expressed as theaverage value (SF (average value)) for 101-151 grains with diametersgreater than 5 mm, and Δθ is expressed as the average value (Δθ (averagevalue)) of the maximum orientation deviations (difference in orientationbetween that at the center of gravity and that at the point furthestfrom the center of gravity in the rolling direction) of 81-113 grains.

[0036] As is clear from FIG. 2, all products satisfying the conditionsof SF (average value)<0.80 and Δθ (average value) (deg)=0.2-4 exhibiteda good magnetic property of W_(17/50) ≦2.00 w/kg.

[0037] To advance their study further, the inventors produced productsmeasuring 0.36-1.00 mm in thickness with slabs, as starting materials,the same as those used in the explanation of FIG. 1 under the sameprocessing conditions as explained with regard to FIG. 1 except that thethickness of the hot-rolled sheets was 2.3-5.0 mm.

[0038] The experimental results for these products are shown in FIG. 3.

[0039] As is clear from FIG. 3, the products satisfying all conditionsof the present invention, namely the conditions of C≦0.0050%, B₈≧1.83 T,SF (average value)<0.80 and Δθ (average value) (deg)=0.2-4, exhibited anexcellent core loss property W_(17/50) of not greater than 3.3×t+0.35(where W_(17/50) is the core loss property in w/kg and t is the productthickness in mm).

[0040] Although the mechanism by which the invention produces its effecthas not been ascertained with complete certainty, the inventors havereached the tentative conclusion set out in the following.

[0041] While core loss property improves with increasing flux density,the improvement is generally diminished in proportion to the extent thatthe increase in magnetic flux density causes the large grain diametersimultaneously. When the sheet thickness is large as in the presentinvention, however, the likelihood of the product grains becomingexcessively large tends to be low. This means that in the case of athick product such as in this invention the correlation between magneticflux density and core loss becomes more clearly defined.

[0042] On the other hand, residual C in the product forms carbides whichprevent movement of the magnetic domain walls during magnetization, thusdegrading the core loss property. In the case of a thick product such asin the present invention, the likelihood of insufficient decarburizationin the decarburization annealing step is high and, therefore,restriction of the product C content is particularly important.

[0043] The basic principle underlying the present invention is that ofachieving a specified combination of product grain boundaryconfiguration and crystal orientation deviation. The tendency for spikemagnetic domains to form in the vicinity of the grain boundaries becomeseven more remarkable when crystal orientation deviation is present inthe grains.

[0044] Moreover, when the irregularity of the grain boundaryconfiguration becomes high (SF becomes low) as in this invention, theresulting enlargement of the grain boundary area increases the frequencyof spike magnetic domain occurrence. The increased number of spikemagnetic domains produced by the invention causes magnetic domainrefinement when the tension is imparted to the sheet by the glass filmand coating, in this way improving the core loss property.

[0045] In the case of a thick sheet as in the present invention, sinceit is difficult to realize a magnetic domain refinement effect only by asimple expedient (such as increasing the tension imparted to the sheet),it becomes necessary to achieve a good core loss property by combininggrain boundary configuration control and in-grain crystal orientationdeviation control as in this invention.

[0046] The reason for the limits placed on the constituent features ofthe invention will now be explained.

[0047] Although there are no particular limits on the composition of theslab used in the invention, in order to stabilize the product magneticflux density and facilitate decarburization to the required level, the Ccontent of the slab is preferably in the range of 0.025-0.075% byweight.

[0048] For achieving improved core loss property, the product sheetaccording to the invention preferably contains 2.5-4.5% Si. Al, N, Mn,S, Se, Sb, B, Cu, Nb, Cr, Sn, Ti, Bi etc. can be added asinhibitor-forming elements. While no particular limit is set on thetemperature at which the slab is heated, energy cost considerations andthe like make it preferable to use a heating temperature of not morethan 1300° C. The heated slab is subjected to hot rolling into ahot-rolled sheet in the following step. The hot-rolled sheet is annealedas required and the sheet is then subjected to a one stage cold rollingor two or more stages of cold rolling with intermediate annealing, forreducing it to the final sheet thickness.

[0049] The reduction ratio in the final cold rolling is not particularlylimited but a reduction ratio of not less than 80% is preferable fromthe point of increasing the product magnetic flux density (B₈ value).Using a reduction ratio of not less than 80% in the final cold rollingensures that the decarburization annealed sheet has appropriate amountsof sharp {110} <001> oriented grains and coincident orientation grains({111} <112> oriented grains or the like) which are likely to be erodedby the {110} <001> oriented grains. This makes it possible to obtain aB₈ of not less than 1.83 T.

[0050] After the final cold rolling, the cold-rolled sheet is subjectedto decarburization annealing at 700-1000° C. Since the product accordingto the invention is thick (0.36-1.00 mm), the time required fordecarburization to the required level tends to be long. For shorteningthe required time, it is helpful to lower the C content of the moltensteel, increase the decarburization annealing temperature, and/or raisethe dew point of the annealing atmosphere.

[0051] That is, preferably, sheets cold-rolled to a final thickness aresubjected to decarburization annealing for 120 seconds to 250 seconds at800° C. to 900° C. in an atmosphere of 25% N₂, 75% H₂ with a dew pointof 60° C. to 75° C.

[0052] If the inhibitor strength is insufficient for evolving secondaryrecrystallization in the decarburized sheet, it is preferable to carryout nitriding treatment using NH₃ gas or some other inhibitorstrengthening measure.

[0053] That is, sheets that have been decarburization-annealed aresubjected to nitriding treatment for 10 to 60 seconds at 7000° C. to900° C. in an atmosphere of dry NH₃ gas to bring the total N content towithin 0.010 to 0.027 weight percent.

[0054] After the sheet has been coated with an annealing separationagent consisting mainly of MgO, it is rolled into a coil having aninside diameter of 10-100,000 mm and then subjected to final finishannealing. When the inside diameter is in this range during finishannealing, the presence of a 0.2-4 deg crystal orientation deviation inrelation to that at the grain center of gravity can be ensured in thesheet grains exceeding 5 mm in diameter.

[0055] The final product is then obtained by subjecting the sheet tostrain relieving and application of a tensile coating. For improving thecore loss property of the product, it is preferable to subject it tomagnetic domain control using a laser beam or the like.

[0056] The final product sheet is required to have an Si content byweight of 2.5-4.5%. At a content below 2.5%, it is hard to obtain a goodcore loss property, while at a content above 4.5% there arises a problemof brittleness during ordinary cold rolling.

[0057] The product according to this invention is thick. Specifically ithas a thickness of 0.36-1.00 mm. A sheet of a thickness of less than0.36 mm may in some cases be able to achieve a good core loss propertywithout satisfying the conditions of this invention. A sheet exceeding1.00 mm is undesirable because the time required for decarburization tothe level required by the invention becomes so long as to cause anintolerable increase in production cost.

[0058] The product has to have a C content of not greater than 0.0050%and a flux density B₈ of not less than 1.83 T. This is because, as shownin FIG. 1, these are the ranges required for obtaining a good core lossproperty. A C content of not more than 0.0030% is preferred.

[0059] The shape factors SF representing the boundary configurationcharacteristics of the sheet grains with the same area as the circlewith diameter exceeding 5 mm has are required to have an average value(an average value for the sheet called the “SF (average value)”) of lessthan 0.80.

[0060] The deviation degree (Δθ) of crystal orientation in grains of adiameter exceeding 5 mm is required to be in the range of Δθ=0.2-4 deg.This is because, as shown in FIG. 2, this is the range required forobtaining good core loss property.

[0061] The invention is not limited to any particular method forcontrolling the SF value and it is possible to select from among controlof the primary recrystallization grain diameter before occurrence ofsecondary recrystallization, use of a grain boundary segregationelements such as Sn, and adjustment of inhibitor strength duringsecondary recrystallization.

[0062] The invention is not limited to any particular method forcontrolling the Δθ value and it is possible either to conduct finalfinish annealing with respect to a coil of a diameter suitable for theproduct grain diameter or to use the heat history between solidificationand slab heating to control the slab grain size. As regards the effectof this Δθ, the presence of the prescribed crystal orientation deviationin even a single grain results in an improvement in core loss property.

[0063] If the foregoing product requirements are satisfied, there isobtained a thick grain-oriented electrical steel sheet exhibiting a goodcore loss property W_(17/50) of not greater than 3.3×t+0.35 (whereW17/50 is the core loss property in w/kg and t is the product thicknessin mm).

[0064] By utilizing the combined effect of the product sheet C contentcontrol, the magnetic flux density control, the grain boundaryconfiguration control and the in-grain crystal orientation deviationcontrol according to this invention, it is possible to obtain a thickgrain-oriented electrical steel sheet exhibiting excellent magneticproperties. The invention can therefore be expected to make a highlysignificant contribution to industry.

EXAMPLES Example 1

[0065] A slab comprising, by weight, 0.053% C, 3.26% Si, 0.15% Mn,0.006% S, 0.029% acid-soluble Al, 0.0076% N and the balance Fe andunavoidable impurities was heated at 1150° C. and then hot rolled into a2.8 mm hot-rolled sheet.

[0066] The hot-rolled sheet was annealed by being held at 1120° C. andthen at 900° C., the annealed sheet was subjected to cold rolling at areduction ratio of about 86% to a thickness of 0.38 mm. One portion ofthe sheet (1) was decarburization-annealed at 800° C. for 150 sec, asecond portion (2) at 830° C. for 150 sec, and a third portion (3) at860° C. for 200 sec, (atmosphere: 25% N₂ and 75% H₂; dew point: 65° C.).The annealed sheets were then subjected to nitriding treatment byannealing at 7500° C. for 30 sec in an annealing atmosphere containingNH₃ gas.

[0067] The N content of the sheets after the nitriding treatment was0.0195-0.0211 wt %. The sheets were then coated with an annealingseparation agent consisting mainly of MgO, rolled into 5-ton coilshaving an inside diameter of 600 mm and then subjected to final finishannealing in which they were heated to 1200° C. at 15° C./hr and held at1200° C. for 20 hr.

[0068] In this final finish annealing, an atmosphere of (25% N₂+75% H₂)was used during the temperature increase phase and an atmosphere of 100%H₂ was used during the 1200° C. holding phase. The coils were thencoated with a tensile coating and cut into SST-sized specimens,flattened, strain-relief annealed at 850° C., and tested for magneticproperties. The final product sheet thickness was 0.40 mm. Table 1 showsthe property values of the sheets treated under the respectiveconditions. FIG. 4 shows the grain pattern of the thick grain-orientedelectrical steel sheet according to the invention. In this figure, Bdenotes the center of gravity of the grain. The crystal orientationdifference between B and A was 3 deg and that between B and C 2.4 deg.TABLE 1 Product Sheet Properties Processing C B₈ Δθ W_(17/50) conditions(%) (T) SF (deg) (w/kg) Remarks (1) 0.0027 1.82 0.82 2.1 1.74 Comparison(2) 0.0020 1.92 0.54 1.2 1.19 Invention (3) 0.0015 1.80 0.50 1.3 1.81Comparison

Example 2

[0069] A first slab (1) comprising 0.045% C, 3.01% Si, 0.14% Mn, 0.008%S, 0.035% acid-soluble Al, 0.0061% N, 0.05% Sn and the balance Fe andunavoidable impurities and a second slab (2) of the same compositionexcept that the Sn content was less than 0.01% were heated at 1150° C.and hot rolled to a thickness of 2.3 mm.

[0070] Without being annealed, the hot-rolled sheets were subjected tocold rolling at a reduction ratio of about 79% to a thickness of 0.48mm. The cold rolled sheets were annealed at 830° C. for 300 sec(atmosphere: 25% N₂ and 75-% H₂; dew point: 62° C.) and were thereaftertreated under the same conditions as those of Example 1. The thicknessof the final product sheets was 0.50 mm. Table 2 shows the propertyvalues of the sheets treated under the respective conditions. TABLE 2Product Sheet Properties Processing C B₈ Δθ W_(17/50) conditions (%) (T)SF (deg) (w/kg) Remarks (1) 0.0020 1.89 0.60 1.5 1.49 Invention (2)0.0015 1.88 0.49 1.1 1.54 Invention

Example 3

[0071] A first slab (1) comprising 0.078% C, 3.21% Si, 0.12% Mn, 0.009%S, 0.034% acid-soluble Al, 0.0060% N and the balance Fe and unavoidableimpurities, a second slab (2) of the same composition except that the Ccontent was 0.053%, and a third slab (3) of the same composition exceptthat the C content was 0.039% were heated at 1200° C. and hot rolled toa thickness of 3.0 mm.

[0072] Without being annealed, the hoc-rolled sheets were subjected tocold rolling at a reduction ratio of about 81% to a thickness of 0.58mm. The cold rolled sheets were annealed at 830° C. for 450 sec(atmosphere: 25% N₂ and 75% H₂; dew point: 62° C.) and were thereaftertreated under the same conditions as those of Example 1. The thicknessof the final product sheets was 0.60 mm.

[0073] Table 3 shows the property values of the sheets treated on therespective conditions. TABLE 3 Product Sheet Properties Processing C B₈Δθ W_(17/50) conditions (%) (T) SF (deg) (w/kg) Remarks (1) 0.0058 1.820.60 1.3 2.41 Comparison (2) 0.0026 1.90 0.57 1.1 1.70 Invention (3)0.0015 1.86 0.65 2.4 1.89 Invention

what is claimed is:
 1. A method for producing a thick grain-orientedelectrical steel sheet with excellent properties, the method comprising;preparing a slab comprising, by weight, 0.025-0.075% of C, 2.5-4.5% ofSi, optionally one or more elements selected from the group consistingof Al, N, Mn, S, Se, Sb, B, Cu, Nb, Cr, Sn, Ti and Bi asinhibitor-forming elements, and the balance being iron and unavoidableimpurities, heating the slab to a temperature not higher than 1,300° C.,hot rolling the slab to a hot-rolled sheet, optionally annealing thehot-rolled sheet, cold rolling the hot-rolled sheet to a cold rolledsheet with a final thickness of 0.36-1.00 mm by a reduction ratio of notless than 80% by using a one stage cold rolling or two or more stages ofcold rolling with intermediate annealing, decarburization annealing thecold-rolled sheet for decarburization of the sheet at a temperaturebetween 700-1,000° C., treating the cold rolled sheet for nitriding byusing NH₃ gas, coating the cold rolled sheet with an annealingseparation agent consisting essentially of MgO, and annealing the coldrolled sheet for final finishing, wherein the thick grain-orientedelectrical steel sheet has a C-content of not greater than 0.0050% byweight, exhibits a magnetic flux density B₈ of not less than 1.83 T andan average value of SF of less than 0.80, where SF is an indexrepresenting the boundary configuration characteristics of theindividual sheet grains with the same area as the circle with diameterexceeding 5 mm has and is defined as SF=(grain area×4π)/(grain boundarylength)², the average value of SF being the average value of theindividual SF values, its grains of a diameter exceeding 5 mm have acrystal orientation deviation of 0.2-4 degrees in relation to that atthe grain center, and the thick grain-oriented electrical steel sheetexhibits a core loss W_(17/50) (w/kg) of not more that 3.3×t+0.35. 2.The method according to claim 1 , comprising: decarburization annealingof cold-rolled sheet for 120 seconds to 250 seconds at 800° C. to 900°C. in an atmosphere of 25% N₂, 75% H₂ with a dew point of 60° C. to 75°C.; and following said decarburization annealing, subjecting the sheetto nitriding treatment for 10 to 60 seconds at 700° C. to 900° C. in anatmosphere of dry NH₃ gas.
 3. The method according to claim 1 whereinfollowing decarburization annealing, the total N content of the sheet isset to 0.010 to 0.027% by weight.